This exhibit is
dedicated to the memory of Mr. Al Swing, who worked for Fluor, a global
engineering firm for many years. Mr. Swing's daily workplace calculator
was a Monroe 1920. On Mr. Swing's passing, his family donated his calculator
to the Old Calculator Museum in honor of his life.

Updated 7/26/2012

The Monroe/Litton 1920 is an interesting
and rather unique scientific calculator. It has a myriad of scientific
functions, and uses a unique way of providing access to its many
unctions with minimal keyboard real-estate, with no use of the
"2nd-function" keys so common on other scientific calculators.
On top of this uniqueness, the machine seems to be a member of a very
small set of desktop, scientific, non-programmable machines from the mid 1970's.
This was a small market segment at the time, because most makers were
concentrating on handheld, battery-powered scientific calculators, or
very complex desktop programmable systems. The 1920 seems to be one
of very few calculators that filled the niche need for a desktop,
non-programmable scientific calculator in the mid 1970's.

This 1920, based on date codes on the Rockwell IC's used to make up the machine,
is from the mid-to-late 1974 timeframe. By this time, IC's were commonplace
in calculators, with basic four-function with memory calculators
all condensed down to one or two chips. However, with the complexity of
operations this machine performs, quite a complement of chips are required
to provide the brains for this machine.

Internals of Monroe 1920

The machine uses seven LSI (Large
Scale Integration) devices to make up its brains. An empty socket for an
eighth LSI indicates that the 1920 was just one in a series of machines
with different functionality available via additional or optional chips.

The Monroe 1930 Statistical ModelPhoto Courtesy Bernadette Fase

Monroe definitely had another machine in the 19x0 series, the 1930, that
looks virtually identical to this machine, but substitutes statistics
calculations for the scientific functions of the 1920. Another hint that there
were various versions of this same general machine comes from the
serial number plate on the bottom of the case. The Model Number
designation has the "19" of 1920 pre-printed, and the "20" part of the
model number is stamped in. All of the LSI
IC's in the machine are made by Rockwell, and have date codes on them that
range from early to mid-1974. The machine is quite highly integrated, with
very few discrete components, with the majority in the power supply, and
a few resistors and capacitors sprinkled here and there amongst the IC's.
The 1920 uses a Burroughs Panaplex II display, which integrates all of
the digits of the display into one display panel. The display module
contains 15 seven-segment digit positions; the left-most digit position
showing a '-' if the number on the display (or mantissa if in scientific
notation) is negative. The next 13 digit positions are used to display
the number (or 10 digits of mantissa if in scientific notation), which
includes commas grouping digits in front of the decimal point into groups
of three. The remaining digits in the display module are used to indicate
the sign and exponent when the display is operating in scientific notation,
with two digits of exponent ranging from -99 to 99. The calculator always
supresses leading and trailing zeros. If the number is too large to fit
in the display using the decimal point position specified, the machine
automatically switches the display to scientific notation.
The calculator displays "Error" using the seven-segment display
elements to form the letters on the display when invalid operations
are attempted, and "OFLO" when an operation results in a number which
exceeds the capacity of the machine. Either error condition results in the
keyboard locking, requiring a press of the [C] key to clear the machine
and unlock the keyboard. The [CD] key clears the display only, useful
for correcting entry errors. The content of memory registers is not
affected by [C] or [CD] keys, however, memory content is lost when
power is removed.

Close up on one of the LSI IC's

The Monroe 1920 has an extensive selection of scientific functions. The
calculator has 10 memory storage registers (0 through 9). The number in the
display can be stored into any of the memory registers by pressing the [STR n]
key, followed by a single digit indicating which memory register the number
is to be stored in. The number in the display can be added to a memory
register by using the [ACC n] key. To recall one of the memory registers,
the [RCL n] key is used, and the number in the specified memory register
is recalled to the display. The machine uses algebraic logic, and includes
parenthesis keys which do allow nesting to up to three levels. The machine
performs sine, cosine, and tangent trig functions,
with [INV] key allowing the arc-versions of each of them; with a three-position
slide switch on the control panel selecting degrees, radians,
or grads for the argument/result of the trig functions. The machine also
can perform natural logarithm, ex (by pressing [INV]
followed by the [LN] key), base 10 logarithm (using the special [II] key with
the [II] key definition switch set to [LOG]), reciprocal (1/x), as well as
raising a number to an abritrary power (ax). Two special
keys in the left-most group of keys, labelled [I] and [II], are 'definable'
keys, whose function at any given time is determined by the position of two
corresponding slide-switches on the control panel labeled "I" and "II". Functions
selectable by these slide switches include recalling the constant Pi,
performing rectangular to polar conversions (and back), performing
degrees/minutes/seconds to decimal notation conversions (and back), base
10 logarithm, special memory clearing options, and various memory summing
options. The exponent of numbers being entered in scientific notation is
entered by pressing the [EXP] key, and the sign of the number in the display
(or exponent) can be toggled using the [CHG SIGN] key. The machine
has an automatic constant in all four basic math functions.

Control switches

The machine can operate in fully-floating decimal point mode(F), scientific
notation(S), an accounting mode using fixed two-digits behind the decimal
point(with cent-sign nomenclature), and some rather unconventional modes
for display of numbers in alternate forms of scientific notation.
The display formatting mode is selected by a single slide-switch at the
center of the switch panel. The [D] key, located above the display format
slide switch, forces the number on the display into the setting on the
display mode switch.

Power Supply & Display construction details

The 1920 uses a regulated linear power
supply. The machine is very solidly constructed, and has a very
high-quality feel to it. The machine is styled purposefully, but
is also quite pleasing to the eye, with a rather stylish swoopy look to it.
The keyboard uses high-quality keycaps with moulded in nomenclature. The
keyboard is made up of magnetically-activated microswitches, and has a
nice feel to it. This machine is quite fast, with no operation consuming
more than one second to complete, and the basic four functions
giving apparently instantaneous answers. During calculation
the display is blanked.